Thanks Eli, Following up on the emacs-devel message, I recompiled emacs
and get substantially the same stack trace. This is with my ~/.emacs and
all my local customizations loaded.

when evaluating a form in a sly lisp buffer with
 (break)
M-x sly-eval-last-sexp
pops up a sly debug window in a new frame
quitting the window (and frame) tries to select the previous window
and the crash apparently happens at this point. I'm attaching the
backtraces as attachments

With a little guidance I expect to be able to investigate this further